Since dust contained in exhaust gases from metallurgical furnaces for iron and steel production, such as blast furnaces, converters, hearth furances and electric furnaces, contains powdered iron oxide in large quantities, it is used as a burden material for blast furances, etc. However, such dust contains also zinc and/or zinc compounds in large quantities. Therefore, it is necessary to remove the zinc contained in said dust while reducing the iron oxide in said dust by firing said dust in the form of pellets in a reducing atmosphere, so that said dust may be used as a burden raw material for blast furnaces, etc.
As a process for satisfying this necessity, a method of producing reduced iron pellets is known which used what is called the grate-kiln system for firing. In this known method, green pellets produced from a material mainly comprising dust, such as blast furnace dust, converter dust, hearth furnace dust, electric furnace dust and/or a mixture thereof, are passed by means of a travelling grate 6' in the direction of arrows in FIG. 1 through a pretreatment furnace 2' comprising a drying zone 5', a preheating zone 4' and a hardening zone 3', where said pellets are dried, preheated and hardened in order, after which said pellets are charged into a rotary kiln and are fired to produce reduced iron pellets. In this process, zinc and/or zinc compounds contained in pellets are removed from said pellets through the high temperature in the rotary kiln and are entrained by the dust in exhaust gases from the rotary kiln. Therefore, the exhaust gases from the rotary kiln contain zinc in large quantities. In the conventional grate-kiln method for firing, hot exhaust gases of a high zinc content from the rotary kiln are used as a heat source for drying, preheating and hardening green pellets to be fired by being introduced into said pretreatment furnace 2'. More specifically, all or part of said hot exhaust gases from the rotary kiln is introduced through the passages shown by dotted lines into the hardening zone 3' of said pretreatment furnace 2' to harden green pellets moving therein, is then introduced through a cyclone 7' into the preheating zone 4' to preheat green pellets moving therein, and after that, is introduced through a cyclone 8' into the drying zone 5' to dry green pellets moving therein, and is finally introduced into a dust collector, such as an electrostatic dust precipitator and a bag filter, where dust in said exhaust gases is collected before said exhaust gases are discharged through a blower 10' out of a stack 13' to open air.
In the conventional method, as mentioned above, hot exhaust gases from the rotary kiln pass through said pretreatment furnace 2' in the order of said hardening zone 3', said preheating zone 4' and then said drying zone 5'. Accordingly, since the temperature of said exhaust gases introduced into said hardening zone 3' and said preheating zone 4' and of green pellets moving in these zones is relatively high, the zinc in said exhaust gases still remains therein. However, the temperature of said exhaust gases introduced into said drying zone 5' has fallen, and the temperature of green pellets moving therein is also low and in a wet condition, whereby part of zinc contained in said exhaust gases deposits on the surface of green pellets therein, resulting in contaminating the pellets with zinc and in reducing the zinc content of said exhaust gases as well. Since pellets so contaminated with zinc must be charged into the rotary kiln, the dezincification rate of the rotary kiln is reduced. Moreover, the zinc content of said exhaust gases is further reduced since said exhaust gases come to contain large quantities of powdered iron oxide produced by bursting of green pellets moving in said drying zone 5' while passing therethrough. Therefore, the zinc content of dust collected by a dust collector 9' is low, thereby making it industrially and economically impossible to use said collected dust as a material for practical zinc refining.
In consideration of the foregoing, though a method is needed for the collection of dust of a high zinc content available as a material for zinc refining in the production of reduced iron pellets by means of the grate-kiln system for firing, such method has not so far been proposed.